In recent years people have become very concerned about exposure to the hazards of microbe contamination. For example, exposure to certain strains of Eschericia coli through the ingestion of undercooked beef can have fatal consequences. Exposure to Salmonella enteritidis through contact with unwashed poultry can cause severe nausea. Mold and yeast (Candida albicans) may cause skin infections. In some instances, biocontamination alters the taste of the food or drink or makes the food unappetizing. With the increased concern by consumers, manufacturers have started to produce products having antimicrobial properties. A wide variety of antimicrobial materials have been developed, which are able to slow or even stop microbial growth; such materials when applied to consumer items may decrease the risk of bacterial infection.
Noble metal ions such as silver and gold ions are known for their antimicrobial properties and have been used in medical care for many years to prevent and treat infection. In recent years, this technology has been applied to consumer products to prevent the transmission of infectious disease and to kill harmful bacteria such as Staphylococcus aureus and Salmonella. In common practice, noble metals, metal ions, metal salts, or compounds containing metal ions having antimicrobial properties may be applied to surfaces to impart an antimicrobial property to the surface. If, or when, the surface is inoculated with harmful microbes, the antimicrobial metal ions or metal complexes, if present in effective concentrations, will slow or even prevent altogether the growth of those microbes. Antimicrobial activity is not limited to noble metals but is also observed in organic materials such as triclosan, and some polymeric materials.
It is important that the antimicrobial active element, molecule, or compound be present on the surface of the article at a concentration sufficient to inhibit microbial growth. This concentration, for a particular antimicrobial agent and bacterium, is often referred to as the minimum inhibitory concentration (or MIC). It is also important that the antimicrobial agent be present on the surface of said article at a concentration significantly below that which may be harmful to the user of said article. This prevents harmful side effects of the article and decreases the risk to the user, while providing the benefit of reducing microbial contamination. More recently, metal ion exchange materials have been developed which are able to effect the so-called “controlled release” of an antimicrobial ion, by virtue of exchange of the antimicrobial ion with ions commonly present in biological environments. This approach is very general since innocuous ions such as sodium and potassium are present in virtually all biological environments. The approach has the advantage in that the antimicrobial ions are bound tightly by the ion exchange medium, but are released when exposed to conditions under which biological growth may occur.
U.S. patent application Ser. No. 0091767 A1 to Podhajny describes a method of applying an antimicrobial treatment to a packaging material, and to polymer dispersions containing antimicrobial zeolites. The zeolite containing dispersions may be formulated in water-based or solvent-based systems. Suitable polymers for practice of the invention listed are polyamides, acrylics, polyvinyl chloride, polymethyl methacrylates, polyurethane, ethyl cellulose, and nitro celluloses.
U.S. Pat. No. 5,556,699 to Niira et al describes transparent polymeric films containing antimicrobial zeolites which are ion exchanged with silver and other ions. The films are said to display antimicrobial properties. Polymeric materials suitable for the invention include ethylene ethyl acrylate (EEA), ethylene vinyl acetate (EVA), polyethylene, polyvinyl chlorides, polyvinyl fluoride resins, and others.
There is a problem in that the polymeric binder or polymeric medium may severely limit the release of the antimicrobial material. Therefore, the exchange of antimicrobial ions from the antimicrobial films may not be facile enough to achieve a concentration of antimicrobial metal ions sufficient to limit the growth rate of a particular microbe, or may not be above the minimum inhibitory concentration (MIC). Alternatively, there is a problem in that the rate of release of antimicrobial ions from antimicrobial films may be too facile, such that the antimicrobial film may quickly be depleted of antimicrobial active materials and become inert or non-functional. Depletion results from rapid diffusion of the active materials into the biological environment with which they are in contact. It is desirable that the rate of release of the antimicrobial ions or molecules be controlled such that the concentration of antimicrobials remains above the MIC. The concentration should remain there over the duration of use of the antimicrobial article. The desired rate of exchange of the antimicrobial may depend upon a number of factors including the identity of the antimicrobial metal ion, the specific microbe to be targeted, and the intended use and duration of use of the antimicrobial article.